Xi: fix wrong extra length check in ProcXIChangeHierarchy (CVE-2017-12178)

Reviewed-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
Reviewed-by: Julien Cristau <jcristau@debian.org>
Signed-off-by: Nathan Kidd <nkidd@opentext.com>
Signed-off-by: Julien Cristau <jcristau@debian.org>
(cherry picked from commit 859b08d5)